1. Field of the Invention
This invention relates to a sensor for measurement of temperature, and more particularly to a temperature sensor using a thermocouple, optimum for use in measurement of temperature of an object heated to a high temperature such as the tip of a soldering iron.
2. Description of the Prior Art
As the sensor for measuring temperature, various sensors are used according to the applications, but it is general to use a thermocouple which is excellent in response where it is necessary to keep at a proper temperature while always checking the present temperature of the object.
For example, in the job for assembling electronic components such as IC in a printed circuit board or the like, soldering is usually necessary, and in this kind of soldering work, the tip temperature of the soldering iron affects the product performance greatly. Accordingly, a high precision is required in the measurement of tip temperature of the soldering iron, and various thermocouples have been proposed recently for used in measurement at high precision as the temperature sensor for the tip of the soldering iron. For example, the thermocouples shown in FIG. 7 and FIG. 8 are typical.
The thermocouple a shown in FIG. 7 is composed of dissimilar metal materials such as chromel and alumel. That is, metal plates b, c (approximately 0.2 mm in thickness 2 to 3 mm in width) composed of these dissimilar metal materials are respectively formed, and the tip part of the metal plates b, c are welded to each other, and mounting parts d, d are provided on their basal parts, and the thermocouple a is formed.
This thermocouple is mounted on the top surface of a recess for measurement e of temperature measuring apparatus, and it is used as the temperature sensor for the tip of the soldering iron.
Besides, the thermocouple f shown in FIG. 8 is composed of dissimilar metal wires g, h made of chromel and alumel mentioned above. That is, both metal wires g, h are formed in an annular shape, and their tips are formed like a cock, and the tips are hooked on each other, so that the thermocouple f is formed.
This thermocouple f is disposed on a base j possessing a recess for measurement k of temperature measuring apparatus, and a weight i is put on one metal wire h to keep tense, so as to be used as the temperature sensor for the tip of the soldering iron.
To measure the tip temperature of the soldering iron by using such temperature sensors, a junction P of the thermocouples a, f shown in FIG. 7, FIG. 8 is used as the measuring point, and the tip 10a of a soldering iron 10 to which solder H is deposited is kept in contact with this measuring point P for several seconds. Then the thermocouples a, f are heated by the tip 10a, and these thermocouples generate thermoelectromotive forces corresponding to the tip temperature of the soldering iron 10. The value of the thermoelectromotive force is converted into a value of temperature by means of analog or digital temperature processing system, and the actual value of the tip temperature of the soldering iron is measured. The measuring precision and response of the temperature at this time greatly depend on the performances of the thermocouples a, f, so that a thinner shape is requested in the former case (FIG. 7), while a thinner metal wire is needed in the latter case (FIG. 8).
In the conventional temperature sensors, however, there were following problems.
(1) The conventional temperature sensors are composed of chromel and alumel, or the like, in the measuring point of the thermocouples a, f, that is, in the junction P, and when the junction P is heated at every time of measurement of tip temperature, the surface of metal materials such as chromel and alumel is oxidized and is darkened, and a film is formed on the junction P. As a result, the tip temperature of the soldering iron 10 is not accurately transmitted to the thermocouples a, f because of this film. This tendency is further intensified as the temperature sensor is used more frequently, which may result in extreme lowering of the measuring precision, and the thermocouples a, f must be replaced early, and the service life is relatively short.
That is, if continued to measure the tip temperature of the soldering iron 10 by using the conventional thermocouples a, f, according to the experiment of the present inventors, as shown in FIG. 10, plotting the number of times of measurement of temperature on the axis of abscissas and the measured temperature on the axis of ordinates, it has been disclosed that a measuring error of about 20.degree. C. is caused when used 50 times, with respect to the tip temperature in the initial period of measurement (the correct tip temperature) of 480.degree. C., and about 47.degree. C. when used 100 times. Thereafter, in proportion to the number of times of use, the measuring error is increased, and therefore the thermocouples were very short in service life, and must be replaced early.
(2) The metal materials used in conventional thermocouples a, f, such as chromel and alumel, are poor in wetting (affinity) with the solder H. Therefore, in the case of, for example, the thermocouple a composed of these metal materials, when the tip 10a of the soldering iron 10 on which solder H is deposited is fitted to the junction P, the solder H becomes like a ball on the surface of the thermocouple a (FIG. 9), and only a point contact is formed between the tip 10a and the junction P.
Therefore, the tip temperature of the soldering iron 10 could not be sufficiently transmitted to the thermocouple a, and still more since the temperature is lowered in the vicinity of the measuring point P, it was difficult to maintain a stable measuring precision.
(3) Since the thermocouple a is in a sheet form, the response can be improved by formingly thinly, but, to the contrary, when becoming thin, the mechanical strength is lowered, and it is hard to endure long-term use.
(4) For fabrication of this thermocouple a, an advanced manufacturing technology is needed, and the product is relatively expensive.
(5) In the case of the latter thermocouple f, the fabrication is relatively easy, but it is necessary to form the metal wires g, h thinly in order to raise the measuring precision. To the contrary, when the metal wires g, h are formed thinly, same as in the case of the above thermocouple a, the mechanical strength is lowered, and the wires are likely to be broken, and it is hard to withstand long-term use, and the life is short, too.